Anode for fused salt electrolysis



Jan. 19, 1960 T. O'CALLAGHAN 2,921,394

ANODE FOR FUSED SALT ELECTROLYSIS Filed May 31, 1957 Z-Sheets-Sheet 2 INVENTOR. F1 5 Th mas oczqnq hc ATTORNEY United States Patent ANODE FOR FUSED SALT ELECTROLYSIS Thomas OCallaghan, Dublin, Ireland, assignor to E. I. du Pont de Nemours and Company, Wilmington, DeL, a corporation of Delaware Application May 31, 1957, Serial No. 669,390 14 Claims. (Cl. 204-243) This invention relates to anodes for electrolysis of fused salts, for example, for the production of sodium by electrolysis of a molten salt mixture containing sodium chloride.

Sodium is commonly produced by electrolysis of a mixture of sodium and calcium chlorides in the molten state, passing a direct current between a graphite anode and a steel cathode. The electrolysis products rise up through the electrolyte along the vertical surfaces of the electrodes and are separately collected. For many years the electrolytic cells utilized for this purpose have been constructed with cylindrical graphite anodes surrounded by circular steel cathodes. This type of cell construction has limited the size and capacity of individual sodium cells. One reason for such limitation is the fact that it is not feasible to obtain or manufacture graphite pieces having maximum, uniform electrical conductvity, sufiiciently large to produce circular graphite anodes larger than about 20 inches in diameter if made from a single piece of graphite. Graphite pieces up to 30 or 40 inches in diameter can be obtained but are somewhat inferior as respects electrical characteristics. The building of exceptionally large cylindrical graphite anodes from a plurality of graphite pieces adds to the cost of the cell and is beset with many practical difiiculties. Further, employment of circular anodes is uneconornical of space, adding to the diificulty of designing cells or groups of cells which will produce a large amount of sodium in a relatively small available floor space. Accordingly, it has been long recognized among the experts in the art, that a cell based upon a rectangular rather than a circular design would be easier and cheaper to construct, would have better operating characteristics and run for a longer time before cell rebuilding is required and would be capable of producing a larger amount of sodium per unit of available factory area than cells utilizing the conventional or cylindrical vertical anodes.

While a rectangular anode design is relatively simple to construct and while rectangular electrodes have long been used in other types of electrolysis, for example in electrolysis of aqueous salt solutions, heretofore the rectangular type of design has not been adaptable for fused salt electrolysis, particularly for large cells carrying 30,- 000 amperes and upwards of current. One reason for this has been that a large rectangular anode must necessarily be made of a plurality of rectangular anode graphite sticks or pieces vertically arranged in a row, with two end pieces and preferably a plurality of middle pieces, and each piece must have a reasonable thickness such as 8 to 20 inches. With such an arrangement it is at once seen that if the anode pieces all have about the same cross-sectional area and electrical conductivity, each can carry about the same amount of current for a given voltage applied to the anode bases. However, the end sticks will have three faces facing the surrounding rectangular cathode while the middle pieces have each only two faces available for electrolysis. Therefore, while each piece carries about the same amount of electric current, the

electrolytic area is considerably greater on the end pieces, with the result that the current density will be much lower on the faces of the end pieces than on the interior pieces. Without the herein disclosed invention, to attain the same current density on the end and middle pieces, it would be necessary to, in some manner, apply a higher voltage to the end pieces. Such is impractical in preferred cell design having current applied at the bottom end of the anode.

An object of the present invention is to provide an improved composite anode of rectangular shape for fused salt electrolysis. A further object is the production of sodium in a fused salt electrolytic cell having rectangular anodes so designed as to maintain at least approximately equal current density on all anodic surfaces. Still other objects will be apparent from the ensuing description of the invention.

The appended drawings illustrate specific examples of the invention. Figure 1 shows in elevation four Vertical graphite sticks or pieces fastened together at their bases to form a rectangular composite anode. Figure 2 is a horizontal cross-section of Figure l in the plane HII. Figure 3 is a vertical cross-section in the plane IIIIII of Figure 1. Figures 4 and 5 represent horizontal cross-sections of end pieces of composite anodes. Figures 6 and 7 are horizontal cross-sections of different types of compositie anodes made in accordance with the present invention.

The above stated objects are attained in accordance with the present invention by so shaping the elongated anode pieces of which the composite rectangular anode is constructed that the ratio of the cross-sectional area of each piece to thte electrolytic perimeter of said piece approximates or is equal to the same ratio of crosssectional area and electrolytic perimeter of every other piece in any plane at right angles to the pieces in the electrolysis zone. The term electrolytic perimeter as used herein means the perimeter of the anode horizontal cross-section. Thus the electrolytic perimeter of each graphite piece forming the composite anode is the sum of the horizontal length of all sides of the piece that face toward the cathode; and the sum of the electrolytic perimeters of all the pieces equals the electrolytic perimeter of the composite anode. The rectangular anode is constructed by clamping together a plurality of vertical rectangular graphite pieces. These graphite pieces may have identical or different cross-sectional shapes but each will have flat sides, so arranged that in the assembled form the flat sides form the four sides of the rectangular composite anode.

In some cases, as shown in Figures 1-3, each graphite piece may be provided with an interior vertical hole or hollow space which communicates with the exterior of the anode by slots or channels. With the slot arrangement as shown in Figures 1, 2 and 3, each piece of graphite is divided into four vertical quadrants each of which carries electric current to the side of electrolysis. Each quadrant then must be considered a separate piece in equating the ratios of cross-sectional areas to electrolytic perimeters.

Referring to Figures 1, 2 and 3, the composite graphite anode is made from four vertical graphite pieces, two end pieces '1 and two middle pieces 2, tightly joined together at the bottom by conventional means not shown. The anode is surrounded by the rectangular steel cathode 3, which defines the electrolysis zone. Direct current is applied to the cathode and to the base of the anode by conventional means. The source of anodic current is applied to the base of each piece of the composite anode, so that the voltage applied is the same for all pieces, e.g. by means of common metallic conductor 9 contacting all the pieces and held in contact therewith by conventional means.

Each graphite piece is provided with a centrally located vertical, cylindrical hole or well 4 and with four slots 5 leading from the exterior to each well. The slots and wells provide for electrolyte circulation. Electrolytic products (e.g. sodium and chlorine) released on the electrode surfaces and floating upwards tend to cause an upward flow of electrolyte in the electrolysis zone. This is compensated'for by a corresponding the wells and around the outside of the cathode, thus continuously feeding fresh electrolyte through the slots 5 into the electrolysis zone.

The slots terminate a short distance from the top of downward flow in 4 2 are larger than those of the end pieces 1. This is necessary in order to maintain the equation because of the relativelyshort length of c.

Reference to Figures 4 and 5 will show the development of the design of the end pieces '1 of Figure 2. Referring to Figure 4, the dimension w is theselected width of the composite anode to-be constructed. :The end piece horizontal cross-sectional dimensions then are w and These selected dimensions maybe varied over a wide range; and the dimension (d-i-e) may be equal the anode, thus forming a solid collar 6, which is desirable to impart greater strengthbut is not essential for the function of the anode. Also, each slot is interrupted about midway'in the electrolysis zone by conductive bridge 7,

which preferably is formed by leaving uncut a small portion of the graphite, as shown.

As shown in the drawing, the graphite pieces are cut away to form a small space between adjacent pieces over the vertical length of the electrolysis zone. This space may be about the same width as the width of the slots or, if desired, a little larger or much smaller. While generally to be preferred, the spaces between the graphite pieces may be eliminated if desired, as they are not essential to this invention.

This arrangement of wells and slots divides each graphite piece into four quadrants; and the amount of electric current carried to the site of electrolysis by each quadrant will be proportional to its cross-sectional'area, as all quadrants in a given piece will have the same specific electrical conductivity, i.e., equal specific resistance;

Because of the relatively high resistance of the electrolyte, as commonly experienced in fused salt electrolysis, practically all of the electrolysis occurs at the graphite faces lying parallel to the opposite cathode face; and the amount of electrolysis occurring within the slots or the spaces between adjacent graphite pieces and below the cathode is so small that it may be neglected in any calculations to account for current distribution.

Now the current density on any face of one of the quadrantswill be in proportion to its cross-sectional area divided by the surface area in the electrolysis zone. Hence, to have the same current density on two quadrants of equal height, the ratio of their respective crosssectional areas to the respective horizontal lengths of the active electrode faces in a given equal. This horizontal length is herein designated the electrolytic perimeter. Thus, the current densities on: quadrants A and B of end piece 1 of Figure 2 will be equal when where A is the cross-sectional area of quadrant A. B is the cross-sectional area of quadrant B.

a is the electrolytic perimeter of quadrant A. b is the electrolytic perimeter of quadrant B.

QLA c a, where:

C is the cross-sectional area of quadrant C. c is the electrolytic perimeter of quadrant C.

horizontal plane must be the middle piece 2 As shown in Figure 2, the wells of the middle pieces to w (square cross-section) or be greateror less. Preferably, the dimensions d and e are selected so that the diagonal lines 8 will divide the cross-sectional area into two area which do not differ greatly in size. The converging diagonal lines 8 of equal length must be drawn to .divide the cross-sectional area of the graphite pieces into two areas A and B such that:

where: a=2d+w; and b=2e 2e r I I w+2d+2e (A +B) From this, B can be calculated. For example, if w=10 inches, =2 inches and e= 10 inches, A'+B"=l20 square inches and V p 20 r I V B X 70.6 square inches and A'=49.4 square inches. Then A B a-r- 5 The placing of lines 8 then is determined by finding the value of the dimension x. Since:

Vertical slots 5 then may be cut in end piece 1 as shown in Figure 5, lines 8 serving as center lines for the slots. The vertical slots may extend through the electrolysis zone up to a short distance from the top of the anode, as shown in Figure 1. The slots divide the piece into two current carrying segments A and B in the electrolysis zone. If desired, two such slotted end pieces may be assembled in back-to-back arrangement as shown in Figure 5, to form a two piece rectangular anode. When placed in use with a rectangular steel cathode like cathode 3 of Figures 1-3, the current density will be uniform over all four; sides of the anode in any given horizontal plane. 7

It is generally preferred to provide the anode pieces with'wells (such as wells 4 of Figure 2) to provide for good electrolyte circulation. The end piece may be provided with such a well 4 as indicated by the broken line circle in Figure 5. The axis ofthe well may be so located that it reduces the areas of A and B in proportion to their respective areas, thus retaining the p'roportions:

where: A and B are the residual areas of A and B after cutting out the well, so that:

5 In actual practice, the axis of the well need not be placed so accurately; that is, the eventual values A B Z and 5- i may differ from each other by as much as about'l0%.

After cutting out the well in such fashion that a-I-b Hence in the above example, where A=49.4, q=14 and 12:20:

nib:-

, 14 7 If the Well diameter is 4 inches, (r=2), then A=44.8 and At this point we have two current carrying segments. Either or both segments A and B may be further divided into two equal quadrants by cutting slots along the center line of the composite anode, as shown in Figure 2 or Figure 6, retaining the value:

a b c where C is the area of a quadrant in a middle piece and c is its electrical perimeter. The radius of the well in a middle piece is calculated from the equation:

a 4 Where: M is the total cross-sectional area of the middle piece 2 before the well is bored, C is the area of one quadrant of the middle piece after the well is bored, and r is the radius of the Well.

Whence:

For example, if the middle piece is of square crosssection, inches square and r=3.3, or the Well is 6.6-.inches in diameter.

In the foregoing calculations, the volume occupied by the slots-has been neglected, as the width of the slots, (e.g., 0.5 to 1 inch for anode pieceslfl and 15 inches square) is so small in proportion that'their neglect does not materially affect the'results;

Again referring to-Figure 4, if desired, lines 8 may be drawn to divide the cross-sectional area of the end piece into two equal areas. In this case:

Whence: x=ed But, in accordance with the present invention:

a V And since in this case A'='=B'; a=b. But a=2d+w; and b=2'e;

Whence: 2d+w=2e, or e.d=% Hence z=g Therefore, regardless of'the'values of dimensions d and e, the distance x will be half the width (w) of the composite anode.

Now, since x=ed, x+2d; (d -l-e), and

d 2 v For example, if the composite anode width is to be 10 inches and the dimensions of the end piece are 10 x 14 inches: A=B'=70 square inches 10 10 inches (d+ e) 14 inches z=- =5 inches e= 9.5 inches Again referring to Figure '2, it should be noted that the slots leading from thewells to the spaces between adjacent graphite pieces may be omitted. .Such modification results in a composite anode illustrated by Figure 6. Each end piece 1 is divided into three current carrying segments, the two identical end segments A and the segment B. Each of the middle pieces 2 is divided into two equal segments C. 7

Another modification of the invention is illustrated by Figure 7. In this case a single well extends throughout the entire length of the composite electrode, terminating in semicircular ends, its width being equal to the diameter of the semicircles. The end pieces 1 are made by boring a circular well centered on the center line of the composite anode, cutting three slots, from the Well to the exterior, one on the composite anode center line and two at right angles on a line passing through the center of the circular well. Two parallel'cuts are then apansei made, tangential to the circular well to complete the rectangular well having a semicircular end.

Three-slots divide the end piece 1 into four segments, ile.,' tvvo pairs of equal volume, and, B. The ratios of, cross-sectional areas to electrolytic perimeter should be equal: i

(I b Now d=d+w, share his half'the'uridthof the composite anode. The perimeter b may be of any desired length;

but the value of dimensiond will depend on the values of b and w and of the radius" f ofthe'semicifcular'c'ut. As

the area 7 I and B=b(w-r), it is evident that A. J w

The area and Whence:

As an example, suppose the composite anode isto be 10 inches wide, the end piece 1 is 10 inches x inches and the selected radius of the circular cut is 2 inches (w=5, r=2 and Applying the above'equations:

d=9.07 inches and b=15d=5.93 inches.

. t Then area A=dwl =42.21 square inches.

And area B=b(w-r)=17.79 square inches.

A 42.21 17 .79' Whence. 7-3, and 3.

The area 0 of the middle piece is c(w-' r) or 30. Hence, the middle piece 2 may be of any desired length and whatever the value of the dimension 0,

with, particularly'iii' relatively narrow composite anodes. 75"

The above described designs insure a uniform current density over the faces of the composite anode in any given horizontal plane, the anode being constructed of a plurality of vertical elongated graphite pieces of equal specific resistance and the same electric current being applied to the bottoms of the pieces. Further, the above described design requires that the cross-sectional area of each piece is substantially non-variant throughout its vertical length in the electrolysis zone. While the current density will be substantially uniform in any given horizontal plane, the resistance of the graphite causes a variation in current density from the bottom to the top of each piece. That is, the current density is a little lower at the top than at the bottom end of the electrolysis zone. This variation which, of course, occurs in the conventional sodium cells, is not of serious import.

When other design factors make it diflicult to meet the equations abc without sacrificing other qualities such as strength, the desired equality of current distribution may be achieved by selecting graphite middle pieces that have specific resistance difierent from that of the end pieces. For example, referring to Figure 2, the wells 4 in the middle pieces 2 can be made smaller if the specific resistance of the middle pieces is sufiiciently greater than that of the endpieees. The proportional amount of electric current carried to the site of electrolysis by separate graphite pieces is dependent upon the specific resistance of each piece. When both pieces have the same specific resistance, the current densities on both will be equal in a given horizontal plane when the ratios of their respective crosss'ectional' areas to the electrolytic perimeters are equal,

or: a

when 1. and C are the cross-sectional areas and a and c are the respective electrolytic perimeters. Now the electric current eifective at the anode faces of segment A of the end piece and segment C of the middle piece is:

. C respectively, where:

, Hence, at equal current densities on A and C:

a a c c A C Whence:

c 777 C wc1rr c 20 where w is the width of the composite anode and r is the radius of the well in the middle piece. Whence:

Lil

pc a we1r1 As seen hereinbefore, when the composite anode width (w) in Figure 6 is 10 and the well in the end piece -1 has a radius of 2,

Then if the dimensions of the middle piece 2 are 10 x 10, :10. If the well in the middle piece piece 2 is made the same size as the well in the end piece (r=2) the area C is:

c, wC11'T 10041r Then Accordingly, if the ratio of the specific resistance of the end piece to that of the middle piece is 1:1.36, the well in the middle piece may be the same size as the end piece well but the current densities on the faces of both will be substantially equal. The greater specific resistance of the middle piece graphite has compensated for the difference in the area/perimeter ratios anda 0 Another feature of the present invention is the provision of bridges 7 (Figures 1 and 3) which interrupt the vertical slots 5. These have a dual function: to add strength to the slotted electrode and further to equalize the electric current in the upper portion of the anode, particularly after a period of use. In use, as in conventional sodium cells, the graphite anode face tends to wear away in the electrolysis zone; and the amount of wear generally is not uniform over the anode face. This results in a change in the proportion of electric current passing through the different segments of the composite anode. This imbalance of current is corrected by the bridge across the slot, which permits current to flow across from one segment to the other.

I claim:

1. In a fused salt electrolytic cell a composite rectangular graphite electrode composed of an assembly of at least three elongated graphite segments arranged in a row and joined to a common source of electric power, so that each segment is adapted separately to carry electric current from said common source to the site of electrolysis, the ratio of the cross-sectional area of said segments to the length of their respective electrode perimeters being approximately equal in any plane at right angles to said segments in the electrolysis zone.

2. An electrode according to claim 1 in which the variation in said ratios of the dilferent pieces is not more than about 3. In a fused salt electrolytic cell, a composite rectangular electrode composed of at least three elongated graphite pieces arranged in a row and connected to a common source of electric current and adapted separatelyto carry electric current into the zone of electrolysis, and having flat faces, the respective crosssectional areas of said pieces and their respective specific electrical resistances being such that the resulting current density is substantially uniform over the rectangular electrolysis zone.

4. An electrode according to claim 3 composed of two vertical end pieces and at least one vertical middle piece, the ratios of the cross-sectional area to electrical perimeter being different for the end pieces and the middle pieces; said end and middle pieces being constructed of graphite having difierent specific resistance, the difference in specific resistance being such that: v

where a and e are the specific resistances of the end and middle pieces, respectively, and

A and g are the ratios of cross-sectional area to electrical perimeter for the end and middle pieces, respectively.

5. An electrode according to claim 3 composed of two vertical end pieces and at least one vertical middle piece, the effective electrical perimeters of the end pieces being greater than those of the middle pieces, the respective cross-sectional areas and specific resistances of all the pieces being such that sufiiciently more electric current is carried by the end pieces to produce on the electrode faces thereof a current density approximating that on the faces of the middle pieces.

6. An electrode according to claim 5 wherein each graphite piece has an interior hollow space extending vertically across at least a major portion of the electrolysis zone and at least one channel leading from said hollow space to the electrolysis zone.

7. An electrode according to claim 6 wherein said graphite pieces are spaced apart from each other and channels also are provided to lead from the interior hollow spaces to the spaces between the graphite pieces.

8. In a fused salt electrolytic cell, a composite rectangular electrode composed of a single row of at least three vertical, elongated, rectangular graphite pieces, fastened together with adjacent pieces in close contact at the bottom ends, having cut away portions so as to form spaces between adjacent pieces in the electrolysis zone, each piece having an interior hollow space extending vertically over the electrolysis zone and vertical slots leading outward from said hollow space to the sides of the composite electrode, so as to divide each piece into segments in the electrolysis zone, means for applying electric current to the joined bases of the graphite pieces, the size and positions of the interior hollow spaces and slots being such that in any horizontal plane in the electrolysis zone the respective ratios of the cross-sectional areas of said segments to their electrical perimeters are approximately equal.

9. An electrode according to claim 8 in which-the variation in said ratios does not vary by more than about 10%.

10. An electrode according to claim 9 in which at least one of the vertical slots is interrupted by at least one electrically conducting bridge serving to equalize current flow in adjacent segments.

11. An electrode according to claim 10 in which the horizontal axes of the slots in all but the two end pieces pass through the centers and are at right angles to the sides and the axes of at least two of the slots in each end piece are at oblique angles to the sides.

12. An electrode according to claim 11 in which the hollow spaces in all but the end pieces are symmetrical and coaxial with the vertical axes of the pieces, the ver- 11. ticall'axes 'of the hollow spaces in the two end spaces liefin the. same plane in the lengthwise center of the composite .electrodebut off-set from the centers of the end pieces, and the specific electrical resistance of the graphite in the end pieces is pieces.

13. A composite electrode according to claim 8 in which electrically conductive bridges are arranged in said slots, adapted to conduct electric current between adjacent segments.

7 l4.- An electrode according to claim 13 in which each piece other than the end pieces is divided into two segments by a single slot'whose' horizontal axis passes through the center of said piece and cuts the lengthdifierent from that in the other wise sides of the electrode at right angles to said lengthwise sides, each of the end pieces is divided into three segments by three vertical slots whose horizontal axes radiate from the interior vertical hollow space, one of said three slots following the horizontal lengthwise axis of the electrode to the end wall which it cuts at right angles, the two other of said three slots cutting the two opposite sidewalls at oblique angles, the hollow spaces in the pieces are cylindrical wells of about equal diameter and the specific resistance of the graphite in the end pieces is difierent from that of the graphite elsewhere in the electrode.

References Cited in the file of this patent UNITED STATES PATENTS 2,213,073 McNitt Aug. 27, 1940 

1. IN A FUSED SALT ELECTROLYTIC CELL A COMPOSITE RECTANGULAR GRAPHITE ELECTRODE COMPOSED OF AN ASSEMBLY OF AT LEAST THREE ELONGATED GRAPHITE SEGMENTS ARRANGED IN A ROW AND JOINED TO A COMMON SOURCE OF ELECTRIC POWER, SO THAT EACH SEGMENT IS ADAPTED SEPARATELY TO CARRY ELECTRIC CURRENT FROM SAID COMMON SOURCE TO THE SITE OF ELECTROLYSIS, THE RATIO OF THE CROSS-SECTIONAL AREA OF SAID SEGMENTS TO THE LENGTH OF THEIR RESPECTIVE ELECTRODE PERIMETERS BEING APPROXIMATELY EQUAL IN ANY PLANE AT RIGHT ANGLES TO SAID SEGMENTS IN THE ELECTROLYSIS ZONE. 